The present invention relates to television timing circuits, and more particularly to a circuit for extracting phase averaged horizontal timing with high noise immunity and rapid signal acquisition from a composite synchronizing signal.
The most critical design in every television production facility is the system timing. There are many sources of video signals in a production facility, such as video camera and video tape recorders, which all must be synchronized so that they may be combined without resulting in the picture on a receiver or monitor rolling, pumping, tearing or having incorrect colors. The necessary timing reference is provided by a sync generator.
The sync generator receives a reference video signal from one video source and generates reference sync signals for the other video sources in the production facility. At the heart of the sync generator is a genlock circuit which locks an internal oscillator loop to the composite sync of the reference video signal. If the reference video source is changed, the new reference video signal may have a composite sync which is either advanced or delayed with respect to the prior reference video signal. Therefore, rapid acquisition of the new signal is required so that the genlock circuit locks to the new reference video signal. However, if the reference video signal is noisy, resulting in jitter of the composite sync, or even loss of some of the sync pulses, it is desirable to maintain genlock on the reference video signal by phase averaging the horizontal sync pulses of the composite sync. These requirements, rapid acquisition and phase averaging, are contradictory. For rapid acquisition it is desirable to lock to the new reference video signal within one horizontal line period, while phase averaging requires waiting for many horizontal lines to assure that the reference video signal has in fact changed rather than been disguised by noise.
What is desired is a sync generator which both time averages the horizontal sync over long periods and locks to new reference video signal phases rapidly.